Process for electrostatic coating of incandescent lamp envelopes



June 1961 G. MEISTER ETAL PROCESS FOR ELECTROSTATIC COATING 0F INCANDESCENT LAMP ENVELOPES Original Filed Jan. 20, 1956 2 Sheets-Sheet 1 Jun 13, 1961 G. MEISTER ETAL 2,988,458 PROCESS FOR ELECTROSTATIC COATING OF INCANDESCENT LAMP ENVELOPES Original Filed Jan. 20, 1956 2 Sheets-Sheet 2 and v m v.

w new w ,H mm 2 w Vim 0 M m n m M A M mg States Patent fiice 2,988,458 Patented June 13, 1961 2,988,458 PROCESS FOR ELECTROSTATIC COATING F INCANDESC-ENI LAMP ENVELOPES George Meister, 82 Vermont Ave., Newark, NJ., and Nicholas F. Cerulli, 73 Forest Ave., Caldwell 62, N .J Orignral application Jan. 20, 1956, Ser. No. 560,441, new Patent No. 2,922,065, dated Jan. 19, 1960. Divided and this application Sept. 30, 1957, Ser. No. 692,954

3 Claims. (Cl. 117-93) This invention relates to incandescent lamps and, more particularly, to a process for applying a diffusingcoating to an incandescent lamp envelope and is a division of application Serial No. 560,441, filed January 20, 1956, titled Incandescent Lamp and now Patent No. 2,922,065 which is a continuation-in-part of application Serial No. 444,316, filed July 19, 1954, titled Incandescent Lamp with Light Diffusing Coating and Method of Manufacture by the co-inventors herein, and now abandoned.

Heretofore commercially-available incandescent lamps with a finely-divided, light-diffusing envelope coating have had a silica coating applied to the lamp envelope by methods as outlined in US. Patent No. 2,545,896 to Pipkin, by flushing processes, or as outlined in US. Patent No. 2,661,438 to Shand.

It is the general object of this invention to provide an improved method for electrostatically applying a finelydivided, light-diffusing material to an incandescent lamp envelope.

FIG. 1 illustrates a silica-coated incandescent lamp;

FIG. 2 represents a first step in the electrostatic coating process;

FIG. 3 represents a coating-material smoke generator;

FIG. 4 illustrates the coating operation for electrostatically applying the coating material to the lamp envelope;

FIG. 5 illustrates the bulb-lehring operation following the coating operation;

FIG. 6 represents the sealing-in operation following bulb lehring;

FIG. 7 is a graph representing observed brightness for various types of silica-coated envelopes vs. distance from neck to top of bulb, -i.e., the candle power distribution for silica-coated envelopes.

Commercially-available silica is normally prepared by precipitating silica from a silicate by means of an acid, for example, by precipitating silica from sodium silicate by means of hydrochloric acid. Such silica is substantially white, porous, generally amorphous and normally inherently spherical in configuration as far as the ultimate particles are concerned. By the descriptive term, generally amorphous, it is meant that X-ray diifraction patterns do not show sharply-defined lines. Also, the porous nature of the silica is another way of stating that the ultimate particles are loosely packed.

In FIG. 1 is illustrated a silica-coated incandescent lamp 20 comprising a vitreous, light-transmitting envelope 22 carrying an internal coating of moisture-conraining, finely-divided silica 24 and having a mount sealed to the neck thereof. A brass or aluminum screw-type base 26 is cemented to the neck to facilitate connection to a power source, as is usual. As is well known, the mount comprises a vitreous re-entrant stern press 28 having lead-in conductors 30 and 32 sealed there-through and supporting a refractory metal filament 34, such as tunge sten, between their inwardly extending extremities. The envelope preferably contains inert gases such as nitrogen, argon, krypton, etc., or mixtures thereof, as is Wellknown, although the lamp may be a vacuum type, if desired.

In electrostatically applying the diifusing silica coating or any other finely-divided, light scattering coating to the inner surface of the unsealed bulb, the open-necked bulb is first placed under and supported by a hollow lava chuck 36, as illustrated in FIG. 2, which chuck cooperates with the bulb cullet 38 and bulb neck 40 to support the bulb. While thus supported, the bulb is rotated either manually or by a belt drive (drive unit not shown) and heated by gas-air burners 42 to approximately C. Because of the negative temperature coefficient of electrical resistance of glass, this heating renders the envelope substantially uniformly electrically-conductive. The heating temperature of 100 C. is given only by way of example and not by way of limitation since the temperature to which the glass is heated to render it substantially uniformly electrically-conductive is not particularly critical and may be varied considerably, according to the type of glass being heated, for example, temperature extrernes of 70 C. to 300 C. have been used, although these temperatures are not intended to be limiting. It should be noted that most incandescent lamp bulbs are fabricated of the Well-known lime glass.

As illustrated, the insulating lava chuck 36 of the electrostati'c coating apparatus is afiixed to a collar 43, which is insulated from the bulb by the chuck, to allow the bulb to be rotated readily, either manually or automatically, during the heating and later steps of the process.

There is illustrated in FIG. 3 a smoke generator unit 44 for producing a smoke of finely-divided particles suspended in air, prior to electrostatic deposition of the powder. The smoke generator comprises generally a powder and smoke reservoir 46 having an outlet 48 at the bottom thereof through which the finely-divided material is admitted into a mixing venturi 50. Compressed air is admitted to the venturi through a pressure-regulating valve 52 and thence to the venturi where the finelydivided material is picked up and carried through a feed conduit 54 to cause the air-particle mixture to impinge upon a target 56 to break-up agglomerates which might have formed and to disperse thoroughly the coating material to form a smoke of finely-divided particles suspended in the air vehicle.

The powder before being placed in the smoke generator unit must be finely-divided and may be ground in an air-velocity type grinder. This breaks up the overlylarge particle agglomerates.

For the specific embodiment of the particle smoke nozzle, which will hereinafter be illustrated and described, it is preferable to maintain a particle-smoke pressure within the reservoir 46 between 6 and 12 pounds during coating to cause the particle-smoke to pass through the smoke nozzle at a desirable velocity. To maintain the smoke pressure in the reservoir within these aforementioned preferred pressure limitations, an indicating gauge 58 is provided from which the operator may have a visual indication so that the pressure-regulating valve 52 may be manually adjusted to maintain the smoke pressure in the reservoir within the aforementioned preferred pressure limitations. Such pressure-indicating and pressure/regulating valves are Well-known. A smokenozzle conduit 60 connects the smoke reservoir with the injector nozzle assembly 62, as shown in FIG. 4. To control the flow of smoke to the nozzle assembly, a manually-operable butterfly valve 64 is provided in the conduit 60.

The air compressor 66 which supplies air to the smokegenerator unit is preferably regulable between 2 lbs. and 20 lbs. output pressure and an air dryer 68, such as a well-known, aluminum-oxide type air dryer is provided in the output line of the compressor so that the particle powder may be maintained under substantially watervapor-free conditions until it is forced into the uncoated bulb. A power-driven agitator 70 (power source not 3 shown) is provided near the base of the reservoir to agitate continually the finely-divided coating material to keep it in a finely-divided state by breaking up the largest particle agglomerates.

The particle powder which is introduced into the reservoir may be a commercially-available grade of silica, for example, which is reasonably pure. Many other finelydivided materials have also been successfully coated. While silica has been found to be the best from a lightscattering aspect, as hereinafter explained, the following materials have been found to be suitable for deposition by the electrostatic process as herein illustrated and described: alkaline-earth and magnesium titanates, stannates, zirconates, oxides, carbonates and silicates; alkaline earth sulphates; titania; zirconia; zinc oxide; alumina; talcs; sodium or calcium alumino-silicates (zeolite); and zirconium silicate. Of course the foregoing materials should be finely-divided (preferably from .02 to slightly more than one micron average diameter ultimate-particle size) to efiectively scatter the light and these particles should appear substantially white under reflected light in order not to absorb excessive amounts of light. The foregoing list of materials is by no means inclusive, but is only illustrative of the multitude of materials which may be electrostatically deposited. It should be noted that as the particles decrease in average, ultimate-particle size, some bluish tint can be seen in reflected light fro-m the coated material, i.e., in the coated lamp when it is unburned. This is apparently due to scattering similar to Rayleigh scattering, to which the blue in the sky is attributed, although the coated material under reflected light only displays a very slight amount of this Rayleigh scattering and cannot be compared to the sky in color. It should also be noted that any finely-divided, substantially-white material normally owes its white color to the fact that the material is actually transparent and the white color is attributed to the light-scattering properties of the fine state of division. An example of this is pure ice, which is transparent, and snow, or finely-divided ice.

In order to control better the moisture content of commercially-available silica, if silica is to be coated, the powder may be baked, if desired, before coating, although the moisture possessed by the silica may be carefully controlled by lehring or baking the coated bulbs after the coating operation and before scaling in and/or exhaust.

In Fig. 4 is shown the coating operation for the bulb. The positive pole 72 of a high-tension, direct-current source is electrically connected to the gas-burner unit 42 and the negative pole 74 is electrically connected to a probe 76 which projects through the hollow-lava chuck 36 into the lower extremities of the bulb neck. If desired these polarities may be reversed with but little effect on the resultant coating. The magnitude of the applied D.C. voltage is not particularly critical and may vary between about 8 kv. and 25 kv., for example, a specific example being kv.

The particle smoke injector nozzle assembly 62 is circumferentially disposed about the probe 76, and the nozzle assembly connects with the nozzle conduit 60 of the smoke generator. The air which is present in the bulb and the particle smoke which do% not deposit on the bulb wall during coating passes through a return conduit 78 which is disposed about the nozzle assembly 62 and the nozzle conduit 60 and which discharges into a collecting hopper (not shown) so as to collect the uncoated particles for reprocessing and further use. A conduit support collar 80 supports the probeand nozzleconduit assemblies and may be positioned longitudinally with respect to the lava chuck and bulb neck either manually or automatically.

It has been found that the particle smoke must be forced through the nozzles of the nozzle assembly in order to coat properly for when the nozzle assembly is removed and the particle smoke is passed through the nozzle conduit into the heated bulb an imperfect coating results. It is thus theorized that the particles when passing through the nozzles are frictionally charged and the application of the high voltage merely attracts the particles carrying an opposite charge to the bulb wall where the charge on the particles is dropped. In support of this, tests were conducted with silica where the applied uni-directional potential was varied between 8 kv. and 25 kv. with no observed difference in the amount of silica which was deposited (i.e., the coating weight on the bulb wall) at the extremes of the 8 kv. to 25 kv. applied voltage. Also, in the case of silica, for example, only about 50% of the powder injected through the nozzle into the bulb is actually coated, the other 50% passing through the return conduit 78.

When the silica smoke, for example, is forced through the nozzles, the particles carry a small net-negative charge, i.e. there are more negatively-charged particles than positively-charged particles. This is perhaps due to the stainless steel of which the nozzle is fabricated and apparently this is responsible for a slightly better powder deposition on the bulb when the bulb wall is made positive with respect to the probe, although very satisfactory coatings can be obtained when the bulb wall is made negative with respect to the probe.

Charging of the particles through static friction be fore application of a high voltage to direct the charged particles to the bulb wall represents a general departure in mechanism from that mechanism which is generally accepted as customary in the electrical-precipitation art. In explanation of this, in the usual electrical precipitation the particles are charged by means of gaseous ions or electrons as opposed to a static-frictional charging through turbulence. The charged particles are then transported to the collecting electrode by the force exerted on the particles by the electric field, and the charged particles are then discharged at the collecting electrode. Thus the electrostatic deposition of this invention apparently dilfers from the conventional electrical precipitation in the particle charging means wherein substantially all particles within the field are deposited.

As a specific example for silica coating a bulb designed for a w. lamp, the nozzle-injector assembly may have eight, evenly spaced nozzles, circurn-ferentially disposed about the probe and each having a diameter of 46 mils. As heretofore noted, the preferred pressure in the smoke generator may vary between 6 and 12 pounds. In coating a bulb adapted for 100 watt operation, the butterfly valve 64 may be opened for about 2 seconds while applying a high tension D.C. of 15 kv. between the bulb wall and the probe. This will deposit approximately 50 mg. of silica onto the bulb. In coating bulb sizes other than the size adapted for 100 watt operation, the number of nozzles which may be used may vary depending on the bulb size. Alternatively, fewer nozzles having a larger diameter or more nozzles having a smaller diameter may be used in the nozzle assembly to coat identical bulbs in order to achieve the same coating result. The nozzle assembly thus constitutes a diffusing orifice which projects charged particles into the heated bulb.

After being coated the bulb is baked or lehred while rotated on the lava chuck, as illustrated in FIG. 5. Bulb lehring is necessary to drive ofi moisture which may have accumulated during coating and in the case of a silica coating, lehring is necessary in order to render the silica coating as moisture possessive or acquisitive as possible. The lehring may be accomplished by gas-air burners 42, as illustrated, and the lehring temperatures may vary considerably depending on the prior processing of the silica coating powder and the conditions under which the processed lamp is intended to operate. For example, if a silica powder is fired before the coating operation at a temperature of about 500 C. for a suflicient time to approach steady-state conditions with regard to moisture content, a bulb lehr of 350 C. for a period of 10 to 20 seconds will normally be suflicient for the silica coating tohave sufii-' cient affinity for moisture to provide an improved lumenmaintenance at normal-operation minimum envelope temperatures, provided the mount is sealed-in, lamp exhausted, gas-fill inserted and exhaust tube tipped-ofi while the bulb is still hot, thus preventing the silica coating from repossessing appreciable amounts of moisture from the atmosphere between the coating, sealing-in and tipping-off operations.

In order to insure adequate moisture-free conditions for the silica coating, particularly where it is desired to operate under high temperature conditions, it is desirable to lehr the coated bulb at from 400 C. to 500 C. for about to 20 seconds and even at this lehring temperature range it is desirable to simultaneously flush the coated lamp with hot, dry air, or other gas at a temperature of about 250 C., for example, to carry away all possible moisture. The air flush temperature is not particularly critical and may vary from about 150 C. to the lehring temperature of the bulb.

If the silica powder has not been baked or fired before the coating operation, higher lehring temperatures preferably are used while simultaneously flushing the bulb with hot dry air, for example, a bake or lehr of 450 C. for about seconds is not considered excessive where a relatively moist, unfired, silica coating must be activated to impart thereto adequate moisture gettering ability for normal lamp operation. This is more completely described in the aforementioned US. Patent No. 2,922,065.

Immediately following the lehring operation and while the bulb is still hot, the mount is sealed in as illustrated in FIG. 6. It is desirable to flush the bulb with hot, dry nitrogen, or other inert gas, while sealing the mount to the bulb neck in order to remove any moisture which may accumulate from the sealing fires, which are normally provided by gas-air burners, as is usual. Such hot, drynitrogen fins-hing is preferably accomplished through the exhaust tube 80 in order to maintain a slight pressure within the bulb to force any moisture out of the neck.

Immediately following the sealing-in operation, and while the bulb portion of the envelope is still hot, the lamp is exhausted through the exhaust tube, the gas-fill inserted and the exhaust tube tipped-01f, as is customary. It may be desirable to further bake the bulb on exhaust to insure that all possible moisture is removed to give the silica coating all possible moisture gettering ability. Baking on exhaust is not absolutely necessary, but is desirable, particularly where the processed lamp is to be operated in hot-recessed or other high-temperature-type fixtures.

After tipping-off, the lamp base is cemented to the neck and the lead-in conductors connected by Well-known lamp basing techniques (basing operation not shown).

The main purpose of a lamp envelope diffusing coating is to diffuse and soften the light emitted by the incandescent filament. Thus the performance of any diffusing coating can be evaluated by the amount of diffusion effected as compared to the percentage of light which is transmitted through the coated, light-diffusing envelope.

There are illustrated in FIG. 7 curves showing the ob served brightness in candles per sq. cm. vs. distance from the neck of the bulb to the top measured in a plane perpendicular to the filament. All filaments used in these measurements were type CC6. It should be noted that the observed brightness measurements represented in FIG. 7 were integrated over a circular area of the lamp surface approximately 0.1 in diameter (roughly 0.008 sq. in.). It is obvious that by increasing the integrated area, hot spots will be smoothed out in the observed brightness measurements, or vice versa, by decreasing the integrated area, hot spots can be intensified. Thus whenever brightness measurements, as illustrated in FIG. 7, are to be evaluated, the area over which such measurements are integrated should be indicated in order that the observed data may be given the proper evaluation. The curves of FIG. 7 may be identified as follows. Curve N represents a standard acid-etch, inside-frost bulb. Curve 0 represents an electrostatically-deposited silica coating on an acid-etched, inside-frost bulb. Curve P represents an electrostaticall-y deposited silica coating on a clear-glass bulb. Curve Q represents a burned ethyl orthosilicate silica coating on an acid-etched, inside-frost bulb. Curve R represents a silica coating on an acid-etched, insidefrost bulb, which coating is applied by spraying onto a hot bulb a silica aquasol containing large particles of silica. Curve S represents a silica coating on an acid-etched, inside-frost bulb, which coating is applied by flushing a 900 C. fired silica powder onto the bulb. As observed, the general shape of all of these curves, with the exception of the inside-frost bulb which is shown for purposes of comparison, is substantially similar and the maximum observed brightness for each type of coated lamp is given in the following table; designated Table I:

Table I Maximum Brightness Lamp Type in Candles Per Sq. Cm.

Acid-etched, inside-frost bulb 28.0 Electrostatically-applied silica coating on inside-frost bulb. 7. 4 Electrostatically-applied silica coating on clear-glass bulb. 7. 2 Blllfi'sd ethyl orthosilieate coating of silica on inside-frost 7 3 u A Silica aquasol containing large si a p hot inside-frost bulb 7. 8 Flush coating of 900 0. fired silica on inside-frost bulb 8. 2

It will be observed that the maximum brightness for the electrostatically-applied silica coatings, both on. clear and inside-frost type bulbs, and the maximum brightness for the burned ethyl orthosilicate coatings when applied to inside-frost bulbs are equivalent. The maximum. brightness for the flush-coated lamps is comparatively greater, i.e., the diffusion efiected by such coatings is somewhat less. The maximum brightness for the silica aquasollarge silica particle coatings on inside-frost bulbs approaches that of the electrostatically-applied silica coatings.

As previously noted, the light-diffusion efliciency for a lamp envelope coating must be measured by the relative transmission efiiciency as well as by the actual diffusion eifected by the coating. It is obvious that a very heavy silica coating will be an excellent diffusing means, although the coating transmission etficiency may be sufiiciently low as to render such coatings relatively poor (i.e., excessively absorptive of light). Thus, in order to evaluate further the foregoing types of coatings, the lighttransmission efficiency for these coatings were tested and the results are given in the following table. In conducting the coating-transmission-efficiency tests, open-necked bulbs were placed over a standard light source in a photometry sphere. A sensitive, linear-responsive photocell, shielded from direct radiation from the standard source, indicated the transmitted-light intensity, which indication is relative to the intensity of the standard source. For example, the standard light source was energized within the sphere and the photocell output noted. This reading represents the value. The bulb whose transmission efliciency was to be measured was placed over the standard light source and the photocell output measurement noted. This photocell measurement was then corrected to indicate a percentage reading as compared to the intensity of the standard light source. The transmission efiiciencies for the heretofore-discussed various types of envelope diffusing coatings were as indicated below in Table II. Also indicated for purposes of comparison are the transmission efiiciencim for clear-glass bulbs and inside-frost bulbs.

Table II Transmis- Lamp Bulb Type sion Elficiency in Percent Standard light source 100 Clear-glass bulb 99 Acid-etch, inside-frost bu 98. 9 Electrostatically-applied sill on clear-glass bulb 97.2 Electrostatically-applied silica coating on inside-frost bulb- 97. 2 Burned ethyl orthosilicate on inside-frost bulb 96. 3 Silica aquasol-large silica particles sprayed on hot inside frost bulb 95. 5 900 0. fired silica powder flushed on inside-frost bulb 96. 7

It will be observed that as compared to a standard acidetched, inside-frost bulb, the electrostatically-applied silica coatings have a transmission efficiency which is 1.7% lower. An equivalent burned ethyl orthosilicate coating on an inside-frost bulb has a transmission efficiency which is 2.6% lower than an inside-frost type bulb. An equivalent silica aquasol-large silica particle, hot-bulbsprayed coating has a transmission efficiency which is 3.4% lower than a standard inside-frost bulb. Thus for an equivalent diffusion (see the foregoing tables listing maximum observed brightness) an electrostatieally-applied silica coating has a transmission efficiency which more nearly approaches a standard acid-etch, inside-frost type bulb than the silica-coated bulbs of the prior art.

It will be recognized that the object of this invention has been achieved by providing an improved method for electrostatically applying a finely divided, light-diffusing material, preferably silica, to an incandescent lamp envelope.

While in accordance with the patent statutes, one bestknown embodiment has been illustrated and described in detail, it is to be particularly understood that the invention is not limited thereto or thereby.

We claim:

1. The method of introducing finely-divided and lightscattering particles through the ditfusing orifice of an orifice-electrode assembly and into an incandescent lamp envelope having an open neck, and coating a substantial portion of such introduced particles onto the interior surfaces of the envelope, which method comprises: heating said envelope to render same substantially uniformly electrically conductive; forming a gas flow path by inserting said orifice-electrode assembly into the open neck of said envelope to form through the diffusing orifice of said assembly a path for gaseous medium entrance into said envelope and to form between said assembly and the neck of said envelope a path for gaseous medium return from said envelope; rapidly rotating said envelope with respect to said orifice-electrode assembly; forcing through the difiusing orifice of said assembly and into said envelope a gaseous medium carrying as a particle smoke an excess of finely-divided and light-scattering particles over that amount desired to be coated with the particle smoke being substantially evenly diffused throughout said envelope as it is forced therein and at least a portion of the particles of the smoke being charged on passing through the orifice of said assembly and into said envelope; simultaneously applying a unidirectional high voltage between the electrode of said assembly and the wall of said envelope to create therebetween an electrostatic field of sufiicient magnitude to cause a substantial portion of the particles introduced into said envelope to be attracted to, adhered to and coated onto substantially all of the interior surface of said envelope; and simultaneously removing the excess uncoated finely-divided particles from said envelope by means of the gaseous medium return flow from said envelope between said assembly and the neck of said envelope.

2. The method as specified in claim 1, wherein the finely'divided and light-scattering particles being coated comprise silica.

3. The method as specified in claim 2, wherein about half of the particles comprising the smoke introduced into said envelope are coated onto the interior surfaces of said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 1,871,367 Hageman et al. Aug. 9, 1932 2,438,561 Kearsley Mar. 30, 1948 2,538,562 Gustin et al. Jan. 16, 1951 2,661,438 Shand Dec. 1, 1953 2,674,973 Thorington Apr. 13, 1954 2,697,025 Fulton et al Dec. 14, 1954 2,806,444 Werner et al. Sept. 17, 1957 2,811,131 Lopenski et a1. Oct. 29, 1957 

1. THE METHOD OF INTRODUCING FINELY-DIVIDED AND LIGHTSCATTERING PARTICLES THROUGH THE DIFFUSING ORIFICE OF AN ORIFICE-ELECTRODE ASSEMBLY AND INTO AN INCANDESCENT LAMP ENVELOPE HAVING AN OPEN NECK, AND COATING A SUBSTANTIAL PORTION OF SUCH INTRODUCED PARTICLES ONTO THE INTERIOR SURFACES OF THE ENVELOPE, WHICH METHOD COMPRISES: HEATING SAID ENVELOPE TO RENDER SAME SUBSTANTIALLY UNIFORMLY ELECTRICALLY CONDUCTIVE, FORMING A GAS FLOW PATH BY INSERTING SAID ORIFICE-ELECTRODE ASSEMBLY INTO THE OPEN NECK OF SAID ENVELOPE TO FORM THROUGH THE DIFFUSING ORIFICE OF SAID ASSEMBLY A PATH FOR GASEOUS MEDIUM ENTRANCE INTO SAID ENVELOPE AND TO FORM BETWEEN SAID ASSEMBLY AND THE NECK OF SAID ENVELOPE A PATH FOR GASEOUS MEDIUM RETURN FROM SAID ENVELOPE, RAPIDLY ROTATING SAID ENVELOPE WITH RESPECT TO SAID ORIFICE-ELECTRODE ASSEMBLY, FORCING THROUGH THE DIFFUSING ORIFICE OF SAID ASSEMBLY AND INTO SAID ENVELOPE A GASEOUS MEDIUM CARRYING AS A PARTICLE SMOKE AN EXCESS OF FINELY-DIVIDED AND LIGHT-SCATTERING PARTICLES OVER THAT AMOUNT DESIRED TO BE COATED WITH THE PARTICLE SMOKE BEING SUBSTANTIALLY EVENLY DIFFUSED THROUGHOUT SAID ENVELOPE AS IT IS FORCED THEREIN AND AT LEAST A PORTION OF THE PARTICLES OF THE SMOKE BEING CHARGED ON PASSING THROUGH THE ORIFICE OF SAID ASSEMBLY AND INTO SAID ENVELOPE, SIMULTANEOUSLY APPLYING A UNIDIRECTIONAL HIGH VOLTAGE BETWEEN THE ELECTRODE OF SAID ASSEMBLY AND THE WALL OF SAID ENVELOPE TO CREATE THEREBETWEEN AN ELECTROSTATIC FIELD OF SUFFICIENT MAGNITUDE TO CAUSE A SUBSTANTIAL PORTION OF THE PARTICLES INTRODUCED INTO SAID ENVELOPE TO BE ATTRACTED TO, ADHERED TO AND COATED ONTO SUBSTANTIALLY ALL OF THE INTERIOR SURFACE OF SAID ENVELOPE, AND SIMULTANEOUSLY REMOVING THE EXCESS UNCOATED FINELY-DIVIDED PARTICLES FROM SAID ENVELOPE BY MEANS OF THE GASEOUS MEDIUM RETURN FLOW FROM SAID ENVELOPE BETWEEN SAID ASSEMBLY AND THE NECK OF SAID ENVELOPE. 